A woman, affected by what activists say was a gas attack, breathes through an oxygen mask inside a field hospital in Hama province in April 2014.
Photograph: Reuters

How has the Assad regime used chemical weapons?

It has deployed them in a variety of ways over the past five years: in grenades and makeshift bombs dropped from helicopters; rockets fired from jets; and artillery rounds and custom made rockets fired from the ground.

Chlorine has frequently been used, but as it has an industrial purpose in Syria it is not banned. Chlorine gas is a crude weapon that can be fatal in high concentrations. In lower doses, it can damage lungs or cause severe breathing difficulties and other symptoms, including vomiting and nausea.

The lethal nerve agent sarin has been deployed less often, but generates more outrage internationally because it can cause widespread loss of life.

Its most recent use was in an attack on the northern town of Khan Sheikhun on 4 April. Earlier attacks took place in villages near Aleppo, Homs, Idlib andthe outskirts of Damascus. A large scale attack involving more than one tonne of sarin killed more than 1,300 people in the suburbs of of the Syrian capital in August 2013. The overwhelming majority of those killed were in opposition communities.

What do we know about the regime’s chemical weapons stocks?

It surrendered most of its stockpile of sarin over a six-month period after the Damascus attack. The process was brokered by Russia, as a means of avoiding reprisal airstrikes on the Assad regime, which had been threatened by the US. The weapons were transported out of Syria by teams organised by the Organisation for the Prohibition of Chemical Weapons (OPCW), which had identified about 1,300 tonnes of sarin and chemicals used to produce it. While much of the stockpile was withdrawn, intelligence agencies believe that up to five tonnes was not declared.

How have past attacks been confirmed?

Biological samples taken from victims was used to confirm the use of sarin in the Damascus and Khan Sheikhun attacks. The tests were performed by OPCW laboratories and government facilities in the UK, the US and France. In the the aftermath of past attacks, intelligence agencies have trawled through intercepted communications, which were not analysed in real time, and found indications that Syrian military units had made preparations ahead of time.

In the Khan Sheikhun attack, preparations included the movement of forces closely linked to the use of chemical weapons to the airbase from where the jet that allegedly dropped the sarin took off.

Those forces included the Syrian Scientific Studies and Research Centre, the Republican Guard and the 4th Division, run by Bashar al-Assad’s brother, Maher. Intelligence agencies also noticed the unusual deployment of air conditioning and air purification units to the same base days earlier. These are used in the preparation and storage of sarin, often when it is mixed and weaponised from binary substances, and typically just before use.